判断图片格式

- (NSString *)contentTypeForImageData:(NSData *)data {

uint8_t c;

[data getBytes:&c length:1];

switch (c) {

case 0xFF:

return @"jpeg";

case 0x89:

return @"png";

case 0x47:

return @"gif";

case 0x49:

case 0x4D:

return @"tiff";

case 0x52:

if ([data length] < 12) {

return nil;
}

NSString *testString = [[NSString alloc] initWithData:[data subdataWithRange:NSMakeRange(0, 12)] encoding:NSASCIIStringEncoding];

if ([testString hasPrefix:@"RIFF"] && [testString hasSuffix:@"WEBP"]) {

return @"webp";
}

return nil;
}

return nil;
}